Vertical and horizontal variability and representativeness of the water vapor isotope composition in the lower troposphere: insight from Ultralight Aircraft flights in southern France during summer 2021

Abstract. The isotopic composition of water vapor is a valuable tool to track atmospheric hydrological processes and to evaluate numerical models simulating the water cycle. To ensure accurate model-observation comparisons, understanding the spatial and temporal distribution of water vapor isotopes in the troposphere is crucial. The challenging task of obtaining highly resolved water vapor isotopic observations is typically addressed through airborne measurements performed onboard conventional aircrafts, but these offer limited microscale insights. This study utilizes observations from ultralight aircraft to examine the water vapor isotopic composition in the lower troposphere of southern France during late summer 2021. By combining the observations with conceptual and numerical models, we identify the main processes driving vertical and spatial variability of isotopic composition and we highlight the detection of short-lived, small-scale processes. The key findings of this study are that (i) at the hourly and sub-daily scales, vertical mixing is the dominant process affecting isotopic variability in the lowermost troposphere and boundary layer above the study site; (ii) evapotranspiration significantly impacts the water vapor isotopic signature, as revealed by the δ¹⁸O-δD relationship; (iii) measurable structures of the water isotopic fields emerge on the scale of 100s of m. The latter are particularly evident for δD, which also exhibit the largest differences in horizontal and vertical gradients. When combined with other airborne datasets, our results support a simple model forced with surface observations to simulate the vertical distribution of tropospheric δD, enhancing the comparison between surface observations and satellite data.